Physics is the natural science that involves the study of matter and its motion and behaviour through space and time, along with related concepts such as energy and force. The main goal of physics is to understand how the universe behaves. In a StackOverflow context physics usually refers to simulating the universe (or some small part of it).

There are several main areas of physics that people commonly try to simulate. Although not an exhaustive list these include:

motion of objects, including changes to the objects because of:

gravitational force from other objects

relativistic speeds

collisions between objects, including changes to the objects because of:

the momentum of the objects

the force imparted by the collision

structural mechanics, including changes to the objects because of:

forces larger than the tensile or compressive strength of the objects involved